BUG: Fix cross-linking to PyGithub documentation

Upstream documentation location for PyGithub has changed; update
intersphinx URL to point to the new location.
ENH: Generate tarball of SlicerWizard doc

Create a new build target to generate a tarball of the SlicerWizard
documentation. This is meant to be used similar to the existing Doxygen
tarball, to be scp'd or otherwise transferred from a build machine to
another machine which will serve the pages publicly.